No icons or items shown in tree view

In a newly started project I made a treeview according the organizations-treeview (conform the demo-project I downloaded). After adding several parent- and child-items no tree-view structure elements were displayed (only the add-button and a yellow bar spanning the screen). Data were correctly stored in the DB (including the relation with dummyContext) and didn't get any errors or warnings. In the demo-project everything works fine and it's certainly no caching-problem. Does anyone have a clue?
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